The invention relates to apparatus for performing measurements of film characteristics (e.g., film endpoint detection and thickness) of a semiconductor wafer during a fabrication process, such as chemical mechanical polishing (CMP) and chemical vapor deposition.
Two approaches to measuring a top-layer film thickness are the four-point probe and scanning electron microscopy (SEM) methods. The four-point method includes forming multiple contacts with the wafer surface to obtain a conductivity measurement. The SEM method includes cross-sectioning the wafer to thereby obtain the film thickness through common SEM imaging techniques. Although the four-point probe and SEM methods may provide adequate film measurements, these tests may only be performed on monitor wafers since these methods destroy the wafer during the measurement process.
One non-destructive measurement approach is to obtain optical measurements of the film thickness, e.g., via optical reflectance or transmission measurements. In-situ optical measurements are typically not performed during a CMP process because the sample undergoing polishing is obscured by debris that may adversely affect the measurement reading. The wafer is polished by rubbing the wafer between a wafer carrier and pad that is atop a platen. A slurry is typically used to mechanically and chemically facilitate removal of a portion of a film deposed on the wafer's surface. The CMP slurry and residues adjacent to the wafer surface are typically optically inhomogeneous and mostly opaque.
This debris (e.g., slurry and film residue) typically interferes with measurements of the sample. In a polishing process, it is desirable to detect when a film has been removed from the wafer, either entirely or to a specific thickness. When the film is removed, this is usually referred to as the endpoint. It is important to detect the endpoint so that the wafer is not over polished. For example, in copper CMP, the copper film is initially optically opaque. Typically, three endpoints are detected in copper CMP. A first endpoint may occur when the copper film is reduced to a specific thickness, which may be, for example, when the copper film begins to become optically transparent. Second, it is determined when the copper is completely removed so that the underlying liner layer (e.g., TaN or WN) is exposed. Finally, it is determined when the liner layer has been removed.
When the endpoint of a film is reached, the polishing can then be stopped without polishing away other structures on the wafer or to change process conditions. Since there is a lot of debris (e.g., slurry and/or film residue) associated with the CMP process, it would be difficult to accurately measure the endpoint while the wafer is undergoing CMP.
Although various approaches to performing in situ optical measurement during CMP have been proposed, none of these approaches solve the problem of debris obscuring the wafer. Of note, U.S. Pat. No. 5,433,651 describes a single beam reflectometer employing a window within a cavity of the CMP polishing pad and platen. The described approach has the disadvantage that CMP slurry and residue can build up in the cavity formed within the platen/polishing pad. The slurry and residue make optical measurements difficult. Another approach, described in E.P. Patent 96302176.1, attempts to solve this problem by providing a “soft window” within the cavity where slurry and residue might otherwise accumulate. Unfortunately, this window typically becomes scratched during the polishing process and pad conditioning and thereby also degrades the quality of optical measurements. Also, the material that is used to form the soft window typically scatters the measuring beam.
U.S. Pat. No. 5,081,796 describes moving a small edge portion of the wafer off the edge of the polishing pad, where the removed portion is then exposed to a jet of water which helps guide a beam onto the wafer's edge. However, this approach has the disadvantage of only measuring the film at the edge of the wafer. Since only a small portion of the entire wafer surface is measured, measurement of the endpoint is not very accurate. Furthermore, this procedure may adversely affect the polishing process.
An alternative approach to performing in situ optical CMP measurements is described in above referenced co-pending Ser. No. 09/396,143 filed Sep. 15, 1999 entitled “APPARATUS AND METHODS FOR PERFORMING SELF-CLEARING OPTICAL MEASUREMENTS” by Nikoonahad et al, which application is incorporated herein by reference in its entirety for all purposes. Although this approach works well for measuring thin films, optical measurements are inadequate for measuring thick films.
Additionally, current approaches for estimating the duration for a film to be removed are inaccurate. That is, the polishing time tends to vary significantly from wafer to wafer. Thus, a significant amount of additional time is added to the polishing time estimate to account for wide variations in polishing time. Although this approach tends to assure that a film will be adequately removed, of course, this approach also adversely affects thorough-put.
Another non-destructive measurement technique utilizes an eddy current probe. One such technique is described in U.S. Pat. No. 6,072,313 by Li et al. This patent describes an eddy current probe that merely detects whether a film has changed. More specifically, the disclosed eddy current probe is formed from a high-Q tuned resonant circuit. This approach has several associated disadvantages. For example, the high-Q resonant circuits are sensitive to environmental changes, and therefore the eddy probe measurements are detrimentally affected by disturbances in environmental conditions, such as temperature, vibration, and changes in distances between the probe and the wafer. Additionally, only magnitude measurements at a single resonant frequency are provided. In sum, present approaches provide a relatively limited amount of information about the film under test.
Accordingly, there is a need for improved in-situ techniques and apparatus for providing information regarding a film while such film is undergoing a deposition or removal process. More specifically, there is a need for non-destructive techniques and apparatus for accurately and efficiently measuring film thickness and/or detecting a film's endpoint.